1. Field of the Invention
The present invention relates to a solid state image pickup apparatus for controlling a charge stored time of a solid state image pickup element in accordance with an incident light intensity.
2. Description of the Prior Art
A technique as disclosed in Japanese Patent Disclosure No. 154880/1981 is known as a method of controlling a charge stored time of a solid image pickup element in accordance with an incident light intensity in order to enlarge the dynamic range of the element.
In this technique, a charge stored time T is controlled in inverse proportion to a mean value L of incident light intensity, as shown in FIG. 1, so that a mean value V of a constant photoelectric transfer output can be always obtained irrespective of the incident light intensity, as shown in FIG. 2. Therefore, the dynamic range of photoelectric transfer is widened, and the configuration of a signal processing circuit can be simplified. For example, when the signal processing circuit performs A/D conversion of a photoelectric transfer output to obtain a digital signal, the level of the photoelectric transfer output must be matched with the input range of the A/D converter. This conventional technique is particularly effective in this respect.
However, when a control unit of this type is assembled in a focusing detection apparatus used in a camera of various type, the incident light intensity, i.e., the brightness of an object to be photographed changes within a very wide range, and the output level cannot be kept constant over such a very wide range of incident light intensity.
As an example, when the incident light intensity is very low, a charge stored time Ts is prolonged, so that the S/N ratio is degraded by a dark current or the like and the response characteristic of the focus detection system is also degraded. For this reason, in order to improve sensitivity, a photoelectric transfer surface of relatively large area is required, and the charge stored time Ts has an upper limit of at best 200 msec.
On the other hand, when the incident light intensity is high, for example, 2.sup.15 times that of the small incident light intensity, the exposure time becomes about Ts=200/2.sup.15 .apprxeq.6 .mu.sec.
In practice, as shown in FIG. 3, it takes a certain period of time (the larger the area of the photoelectric transfer surface and charge storage section, the longer the transfer time; i.e., several microseconds to several tens of microseconds) for a photoelectric transfer charge to be completely transferred to a CCD shift register. For this reason, the charge generated in the photoelectric transfer surface exposed during such a certain period of time is also transferred to the CCD shift register, thereby causing an exposure error. Then, the photoelectric transfer output increases as indicated by the broken line shown in FIG. 2, so that an output is not suitable for input to the signal processing circuit. This has limited the dynamic range.